- THE MAGAZINE
Magnetron sputtering, combined with an accurate control of process parameters and layer quality, has become one of the most important methods for depositing thin films. The technique involves bombarding a target surface, which is positioned on a magnetic tube, with an ionized gas. The gas causes metallic atoms to be ejected from the target and subsequently deposited on the substrate to be coated. In standard metallic sputtering, an inert gas, such as argon, is used. No chemical reaction occurs between the gas and the target particles, resulting in a coating on the substrate with a composition similar to the target material.
In a reactive sputtering process, at least one reactive gas (e.g., oxygen or nitrogen) is added. The reactive gas enhances the sputtering process on the target surface and also generates a chemical reaction with the target particles, forming a compound layer on the substrate. As a result, high-purity, uniform coatings can be achieved.
A device called the rotating cylindrical magnetron can overcome all of these problems. By implementing new improvements to the magnet configuration, the technology can achieve target utilization levels of more than 90%. Local tuning of the magnetic field strength allows the layer thickness to be accurately controlled over the substrate width, and new technological developments to overcome arcing and anode problems have also been implemented in this system. With rotating cylindrical magnetrons, companies can achieve controlled reactive sputtering on a wide variety of substrates.
Equipment DesignTwo rotating cylindrical magnetron designs have been used for both metallic and reactive sputtering processes. In the first concept, the cylindrical target tube is supported and powered by two end blocks—one block supplies the target cooling and electrical power to the target tube, while the other sustains the rotation. The latter end block has a spindle that can be retracted very easily, allowing the target tube to be quickly removed for maintenance or replacement. The end blocks are mounted inside a vacuum chamber along with the target. With this robust configuration, targets up to 13 ft (4 m) long can be mounted. Figure 1 shows a double end block set configuration for coating large-area glass, with two 152-in. tin (Sn) target tubes mounted.
In both concepts, the key component is the rotating target tube, which contains a stationary magnet configuration (defining a stationary plasma racetrack) facing the substrate. The magnet configuration can be optimized at the target ends to extend the width of the uniform coating. The magnetic field strength and the corresponding layer thickness can also be easily adjusted.
Arcing and Anode AdvantagesDuring the reactive sputter deposition process, the insulating compound layer is formed not only on the substrate, but also on the chamber walls, on the anodes and on the target surface next to the erosion zone. On the target surface, the insulating layer charges up by ionic bombardment; when the dielectric strength is exceeded, electrical breakdown occurs, leading to arc discharges (or arcing). Arcing can cause process destabilization and physical damage to the target, anodes and substrate.
Initially, lower power levels were used to reduce the influence of arcing. Power supplies were later equipped with arc detection and interruptible circuitry (i.e., “pulsing”) to limit the amount of energy dissipated into the arc. However, neither of these technologies has been capable of completely eliminating arcing in the racetrack area of the target.
The introduction of large rotating cylindrical targets has restricted the arcing zone to two ring-shaped areas at the cathode tube ends.1,2 Very little arcing occurs in the racetrack area of a rotating cylindrical target, since it is continuously cleaned by the plasma. In fact, as long as the target rotation speed is high enough, only a thin zone at both extreme ends of the target tubes gets contaminated with the insulating material. (If the speed is too low, contamination of the complete target surface can occur before it is sputter-cleaned again, leading to arc events.) A setting of 10 rpm is sufficient for most reactive processes.
Finally, rotating cylindrical magnetrons have also proven to be useful for applications that require AC sputtering from two targets. The AC switching technology was introduced to circumvent the problems caused by arc discharges (by periodically discharging the cathode) and the disappearing anode problem (by sputter cleaning the target, which becomes an anode in the next cycle). However, on planar magnetrons, the magnetic field above the target surface that is used to enhance the plasma density impedes the flow of electrons to the target, resulting in a limited anode functionally. In rotating cylindrical magnetrons, the magnetic field-free area and the total effective surface are much larger, resulting in enhanced anode functionality.
Bottom Line BenefitsFor planar magnetrons, the presence of a racetrack groove limits target consumption, thereby increasing the cost of operating the technology. Although a stationary plasma track is also present during sputter deposition with a rotating cylindrical target, no racetrack groove corresponding to the magnet configuration is formed in the rotating target. As a result, very high target material utilization can be achieved.
Although the target utilization of cylindrical magnetrons is several times higher than a planar magnetron, up to 40% of that utilization can be gained or lost by the target and magnet setup at the end of the target tube. Using a dog bone-shaped target rather than a straight target can help increase target consumption (see Figure 4). Additionally, by strategically placing additional magnets on the target tube to provide a “double racetrack,” in which the turns are slightly shifted, virtually complete target consumption (up to 90%) can be achieved (see Figure 5).
In addition to providing increased target consumption efficiency, rotating cylindrical magnetrons also enhance the production process in several other ways. In some coating applications, higher deposition rates are preferred. This requires both a higher machine throughput (achieved through higher power levels) and an increased coating capacity. For planar targets, the use of higher power levels is not always possible because the target material can melt or crack under the thermal load. In rotating cylindrical targets, the thermal load is uniformly distributed over the complete circumference of the target, resulting in much more efficient cooling of the target tube and the ability to use higher power densities.
Rotating cylindrical targets also offer a higher coating capacity compared to a similar standard planar target. This is due to the higher target utilization of the rotating cylindrical target, as well as the larger amount of available target material. (In some cases, the available target material is up to three times larger because of the circumferential design.) As a result, the life of the cylindrical target is often five to 10 times higher than with a planar magnetron.
Optimized SputteringThe rotating cylindrical magnetron provides a valuable alternative to planar magnetrons. Because the target tube rotates over a stationary closed-loop racetrack, the system overcomes most of the shortcomings associated with the typical erosion groove formation in planar magnetrons. Almost complete target consumption can be achieved by optimizing the magnet bar configuration. The system offers a reduced arc rate and better anode functionality in an AC switching mode, and it can also generate higher power densities and sputter yields because the heat load is uniformly spread over the complete target tube surface. Finally, since a tubular target contains about three times more material than a planar target for the same width, the target lifetime or coating volume can be five to 10 times higher compared to planar magnetrons.
With the rotating cylindrical magnetron, high-quality coatings can be applied easily and cost-effectively even in large-area and high-volume applications, giving glass and ceramic manufacturers new options for coating optimization.
References:1. De Bosscher, W.; Gobin, G.; and De Gryse, R., Proc. 3rd Int. Conf. On Coatings on Glass (2000), p. 59.
2. Blondeel, A. and De Bosscher, W., SVC 44rd ATCP (2001), p. 240.